Static and dynamic X Y -like short-range order in a frustrated magnet with exchange disorder

Abstract

A single crystal of the Co2+-based pyrochlore NaCaCo2F7 was studied by inelastic neutron scattering. This frustrated magnet with quenched exchange disorder remains in a strongly correlated paramagnetic state down to 1/60th of the Curie-Weiss temperature. Below Tf=2.4 K, diffuse elastic scattering develops and comprises 30±10% of the total magnetic scattering, as expected for Jeff=1/2 moments frozen on a time scale that exceeds /δE=3.8 ps. The diffuse scattering is consistent with short range XY antiferromagnetism with a correlation length of 16 Å. The momentum (Q) dependence of the inelastic intensity indicates relaxing XY-like antiferromagnetic clusters at energies below ∼5.5 meV, and collinear antiferromagnetic fluctuations above this energy. The relevant XY configurations form a continuous manifold of symmetry-related states. Contrary to well-known models that produce this continuous manifold, order-by-disorder does not select an ordered state in NaCaCo2F7 despite evidence for weak (∼12%) exchange disorder. Instead, NaCaCo2F7 freezes into short range ordered clusters that span this manifold. © 2016 American Physical Society.